Ultraviolet radiation causes a chain of chemical reactions in the ice, the nature of which remained a mystery for decades. Now researchers at the University of Chicago and the Abdus Salam International Center for Theoretical Physics have used quantum mechanical modeling to show that microscopic defects in the ice crystal lattice play a key role. The work was published in the journal Proceedings of the National Academy of Sciences (PNAS).
The first strange thing was noticed nearly 40 years ago: ice samples exposed to UV light for a few minutes and samples exposed to UV light for hours absorbed different wavelengths. This means that the chemical properties of ice change under the influence of light, but then these processes cannot be explained.
“No one has before been able to model the interaction of UV light and ice with such precision,” said Julia Galli, a professor of molecular engineering at the University of Chicago.
The team applied computational techniques that Galli and her colleagues were developing to study quantum technology materials. These methods make it possible to “split” the ice into atomic processes and to observe precisely how defects affect the interaction with ultraviolet light – something that cannot be seen directly in experiments.
Marta Monti, lead author of the work, explains: “Ice is an extremely complex object. When interacting with light, water molecules can break down, forming new radicals and ions, and these products completely change the behavior of the material.”
Scientists studied four types of ice: a perfect crystal and three variants with defects – vacancies (missing molecules), embedded hydroxide ions and Bjerrum defects, which disrupt the hydrogen bond order. Each type of defect radically changes the energy at which the ice begins to absorb UV light, leaving a unique “optical fingerprint” that allows it to be identified in real samples and influences how electrons in the ice behave when irradiated – whether they move freely or are trapped in microscopic cavities.
These results explain observations in the 1980s when ice, after long periods of irradiation, exhibited new absorption lines that were previously unexplained.
Understanding how ice absorbs and emits light will help model melting, crack formation, and interactions between ice and the atmosphere.
The team has prepared experiments to directly test the model's predictions and plans to study more complex defect sets as well as the influence of the meltwater layer.
